Nontrivial Topological Surface States in SmB$_{\mathrm{6}}$ Thin Films

Being identified as the first rare earth mixed valence system and the first Kondo insulator, SmB$_{\mathrm{6}}$ may very likely be the first topological Kondo insulator as well. Recent studies, theoretical and experimental, have suggested the existence of metallic surface states in single-crystal SmB$_{\mathrm{6}}$ bulk materials, but the presumed topologically nontrivial nature and spatial scale of the surface states, as well as many other aspects, remain outstanding. This work demonstrates the nontrivial topological nature of the surface states in SmB$_{\mathrm{6}}$ thin films via a spin pumping technique with samples of SmB$_{\mathrm{6}}$ thin films grown on Y$_{\mathrm{3}}$Fe$_{\mathrm{5}}$O$_{\mathrm{12}}$ (YIG) slabs. In samples where the SmB$_{\mathrm{6}}$ film thickness ($d)$ is 80 nm or larger, the spin-pumping voltage signal becomes much stronger as the temperature ($T)$ decreases from 150 K to 10 K. Such an enhancement originates from spin-momentum locking of the metallic surface states, thereby providing strong evidence for the nontrivial topological nature of the surface states. The results also suggest a thickness of about 32 nm for the topological surface state, which was confirmed by $T$-dependent transport measurements and theoretical analysis using the tight binding model.